Compact pressure-sensing device

ABSTRACT

The invention is a compact sensing device that is capable of measuring the conditions (e,g, pressure, temperature) inside a cylinder of an internal combustion engine. The invention is also a cost-effective method of fabricating the sensing device. The sensing device includes a substrate, a beam, and piezo-resistive sensing elements. The beam, which is formed on the substrate, is capable of deflecting according to different pressures applied to different beam surfaces. The piezo-resistive sensing elements are coupled to the beam and detect beam deflection. The piezo-resistive sensing elements generate an electrical signal corresponding to the beam deflection.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application claims the benefit of priority under 35 U.S.C. §119(e)of U.S. Ser. No. 60/669,746 filed on Apr. 8, 2005, the entire content ofwhich is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to a sensor and more specifically to aminiature sensor with good heat tolerance.

2. Background Information

Internal combustion engines are relied upon today for numerousapplications, primarily transportation. For the engines to performreliably at the desired performance level and last its potentiallifespan, they need periodic maintenance. Engine monitoring systems areused to determine when a maintenance service is due for an engine.Today, most engine monitoring systems make this determination bymonitoring the exhaust output and the time interval since its lastmaintenance/service.

Although these engine monitoring systems detect service needs wellenough in many situations, there is room for improvement. Onedisadvantage of the current engine monitoring system is that thevehicles using the engine usually incorporate an adaptive control whichcan compensate for wear of the pistons, cylinders, and valves to acertain degree. Due to the function of the adaptive control, it possiblefor unacceptable wear and catastrophic failure of the engine to occur“suddenly” without sufficient warning.

This undesirable “sudden” failure can be avoided by monitoring thecombustion processes inside the cylinder where pressure loss could bedetected to provide accurate, undisguised status of the engine. However,although it is known that in-cylinder pressure and temperature sensorsystems are highly desirable, they are too costly to implement. Thechallenges in implementing in-cylinder sensor systems stem from theexpected life span of the sensors and the initial cost and the necessityto bore, tap, and die a fixing into the cylinder head for the sensor.The cost and complexity of fitting a pressure sensor into the cylinderhead is inhibitive despite its advantages. Thus, attempts to measure theenvironmental factors (e.g., temperature and pressure) in the cylinderinvolve placing sensor arrays outside the combustion cylinder.

Generally, there are two types of sensors: sealed cavity and optical.The sealed cavity-type uses MEMS technology to micro machine out thecavity and use either capacitive or piezo-resistive techniques tomeasure the deflection of a diaphragm over the cavity as the ambientpressure changes in relation to that in the cavity. The optical-typesensors use either a cavity technique to bounce light off of thediaphragm or interferometry to gauge the strain in a fiber optic cable.Both types of sensors require access to the sensor by boring through thecylinder head and are of relatively high unit cost. Due to the highcost, these sensors have thus been limited to laboratory-scaleapplications.

Thus, a need exists for a cost-effective means of accurately monitoringenvironmental factors in a combustion cylinder.

SUMMARY OF THE INVENTION

The invention is a sensing device that is capable of measuring theconditions (e.g., pressure, temperature) inside a cylinder of aninternal combustion engine. The invention is also a cost-effectivemethod of fabricating the sensing device.

In one aspect, the invention is a sensing device that includes asubstrate, a beam, and piezo-resistive sensing elements. The beam, whichis formed on the substrate, is capable of deflecting according todifferent pressures applied to different beam surfaces. Thepiezo-resistive sensing elements are coupled to the beam and detect beamdeflection. The piezo-resistive sensing elements generate an electricalsignal corresponding to the beam deflection.

In another aspect, the invention is a sensor that includes a flexiblematerial and a sensing device mounted on the flexible material. Theflexible material has an electrical interface. The sensing deviceincludes a substrate, a cavity formed in the substrate, a beamed formedon the substrate, piezo-resistive sensing elements coupled to the beam,and bond pads electrically coupled to the piezo-resistive sensingelements. The beam is capable of deflecting according to differencebetween pressure inside the cavity and pressure outside the cavity. Thepiezo-resistive sensing elements detect beam deflection and generate anelectrical signal corresponding to the beam deflection. The bond padsare electrically coupled to the electrical interface on the flexiblematerial.

In another aspect, the invention is a method of fabricating a sensingdevice. The method entails providing a substrate, forming a beam on thesubstrate, and doping a portion of the substrate to form piezo-resistivesensing elements coupled to the beam. The piezo-resistive sensingelements detect beam deflection and generate an electrical signalcorresponding to the beam deflection.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic of a sensor device in accordance with anembodiment of the invention;

FIG. 2A is a diagram of a beam with uniform cross-section and both endsfixed;

FIGS. 2B and 2C are plots showing the moment and the displacement,respectively, of the beam of FIG. 2A along the length of the beam;

FIG. 3A is a diagram of a beam with uniform cross-section having onefixed end and one free end;

FIGS. 3B and 3C are plots showing the moment and the displacement,respectively, of the beam of FIG. 3A along the length of the beam;

FIG. 4A is a diagram of a beam with decreasing cross-section having onefixed end and one free end;

FIGS. 4B and 4C are plots showing the moment and the displacement,respectively, of the beam of FIG. 4A along the length of the beam;

FIG. 5A is a diagram of a beam with increasing cross-section having onefixed end and one free end;

FIGS. 5B and 5C are plots showing the moment and the displacement,respectively, of the beam of FIG. 5A along the length of the beam;

FIG. 6 is a plot showing typical pressure and burn rate for an internalcombustion engine;

FIG. 7 is a plot of a digitized pressure curve;

FIG. 8 is a plot of digitized burn rate and burn amount;

FIG. 9 is a schematic of the sensing device fitted between the gasket;

FIG. 10 is a sensing device having a fixed-free beam;

FIG. 11 is a sensing device having a fixed-fixed beam;

FIGS. 12 and 13 are a fixed-fixed beam having a width of 100 μm and 1000μm, respectively;

FIGS. 14 through 20 are masks that may be used for fabrication of asensing device with a fixed-free beam;

FIG. 21 is an optional test cell that may be fabricated with the sensingdevice on the same mask set;

FIG. 22 is a schematic depiction of sensing device fabrication process;

FIG. 23 is an exemplary flex material that may be used to interface thesensing device; and

FIG. 24 is a bridge circuit that may be used to measure beam deflection.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a combined MEMS pressure and temperature sensorfor measuring the realtime combustion conditions of each cylinder in aninternal combustion engine. In one embodiment, the device can befabricated as a roughly planar micro-machined sensor of 20 μm (0.0008″)thickness using piezo resistors to measure both the strain andtemperature. The device can use a cavity flow method of pressuremeasurement.

The invention solves technical and economic problems with sensors thatare currently available. An advantage of the invention is that thedevice is roughly planar and less than approximately 100 μm thickincluding packaging, therefore able to fit as part of the cylinder headgasket without requiring machining or redesigning of the cylinder head.The device is therefore able to be fitted to units currently in serviceas well as new units. The thinness of the device will also allow it tobe easily transferred over to other applications where a gasket is usedto mate components such as gas turbines. Additionally, the device of theinvention may be made with a MEMS construction technique that allows forcost-effective mass production with good homogeneity. The use of MEMStechnology, and in particular silicon and silicon carbide, allows thefabrication of a device that offers high performance and low dimensions.

The device brings significant improvement in engine monitoring, as itmeasures the actual conditions in the cylinder. Additionally, combustionpressure and temperature data can be used to improve performancereducing the need for many other sensors that are currently used on theperiphery of the engine.

Device Geometry

FIG. 1 shows a sensing device 10 in accordance with the invention. Thesensing device 10 is compact, being no larger than about 10 cm×10 cm×100μm in size and sometimes as small as 10 cm×10 cm×20 μm. The sensingdevice 10 has a cavity 12 that acts as a pressure chamber and a beam 14that moves in the plane of the sensing device 10. As the pressure in thespace around the sensing device 10 increases, gas flows into the cavity12 through a gas flow passage 16, causing the beam 14 to deflect intothe cavity 12. After a certain period of gas flow (after a cylindercycle), the pressure outside the sensing device 10 decreases as thepost-combustion gases exit the cylinder. At this point, the beam 14returns to neutral position. To ensure free movement of the beam 14, itsdimensions in the plane of the device (i.e., the thickness) are reducedon the side that it contacts the gasket. Computational analysis can beemployed to calculate the deflection of the metal (inner cylinder) sealover the device.

The beam 14 is coupled to aluminum tracks 18 that lead to bond pads 20.At the interface between the aluminum tracks 18 and the beam 14 arepiezo-resistive sensing elements 22. These sensing elements 22 areformed by being doped into a silicon substrate. The beam 14 isstructured to induce maximum strain at its base where thepiezo-resistive sensing elements 22 are doped into the silicon. Thepiezo-resistive sensing elements 22 detect the position of the beam 14,which changes to reflect the pressure in the surroundings as describedabove.

At the same crystal orientation, two sets of resistors can be placed inthe bulk (un-strained) silicon to form additional piezo-resistivesensing elements 22 for temperature detection. With two sets ofpressure-measuring sensing elements and two sets of temperature sensingelements, an electronic bridge can be set up to read the effect ofstrain on the device. The sensing device 10 may be designed with fourresistors not wired together. The resistors may be wired to form atwo-resistor bridge or a four-resistor bridge. In the case of thetwo-resistor bridge, the remaining two resistors are used fortemperature compensation. In the four resistor bridge, no temperaturecompensation is done because the resistors are so close to one anotherto need temperature compensation.

In some embodiments, a single piezo resistor is placed at the base ofthe sensing device 10, in addition to the pair of pressure sensingelements 22, for temperature measurement. In other embodiments, a p-ndiode may be used instead of the single piezo resistor. In theseembodiments, the combination of photons and phonons from the combustionprocess may provide the diode with a “finger print” identification of agood or bad combustion process.

The sensing device 10 may be placed approximately 10 mm off the cylinderwhere temperatures are lower due to engine cooling. The sensing device10 is implemented by being mounted on a flex material, as will bedescribed below, and the cooler temperature 10 mm away from the cylinderfacilitates the design of the flex material. The flex material has to beable to tolerate the expected maximum temperature in the area where thesensing device 10 is placed. For the sensing device 10 to operateeffectively, the sensor (i.e., the sensing device 10 mounted on a flexmaterial) is mated to the cylinder head gasket, allowing free movementof the beam 14. A secure mating is achieved by micro-machiningtechniques but the plastic flow of the seal ring on the head gasket maypose an issue. Thus, gasket designs to accommodate micro-sensors may beimplemented, concentrating on even distribution of clamping forcesaround the sensor. Preferably, the clamping force is sufficient to sealthe cylinder during firing but loose enough to allow free beam movement.

Device Design

The sensing device 10 may be fabricated by using a MEMS process. In thefabrication process, flow rates of gas through the gas flow passage 16is calculated. The calculation result is then used to evaluate thepressure difference between the space in the cylinder and the cavity 12.The geometries of the beam 14 and the plastic hinge where thepiezo-resistive sensors 22 are mounted are determined based oncomputation analysis using the pressure difference. The anticipatedstrain during firing can then be determined and the results are used toset up the electronics and signal analysis.

Specifically, a mathematical model is used to determine the geometry ofthe beam 14 and the plastic hinge. The mathematical model uses fluiddynamics to calculate the flow rate through an orifice (gas flow passage16) into the cavity 12. The flow rate is in turn used to determine thepressure difference between the cylinder and the cavity 12. In someembodiments, the beam 14 is designed to have a lower section modulus atits base, thus maximizing the stresses at the base. The well-knownbending theory is used to calculate this stress and ensure that it doesnot exceed the Ultimate Tensile Strength (UTS) of the silicon. The goalis to maximize the strain at the base of the beam 14 where the two piezoresistive sensing elements 22 are placed. The piezo resistive sensingelements 22 are used to calculate the pressure in the cylinder.

When designing the sensing device 10, cavity resonations and the basicresponse time of the cavity orifice (gas flow passage 16) are taken intoaccount.

The parameter for cavity resonator comes into play when a small space(such as the cavity 12) is connected to a larger space (such as thecylinder) by connecting lines that have a volume less than the cavity.The smaller space can support acoustic oscillations between it and thelarger space. The dynamic equations for the gas flow in the connectinglines can be expressed as follows:

${\left( {p - p_{a}} \right)A_{o}} = {\rho\; A_{o}L_{o}\frac{\mathbb{d}u}{\mathbb{d}t}\mspace{14mu}{Orifice}\mspace{14mu}{Momemtum}}$${V\frac{\partial\rho}{\partial t}} = {{- \rho}\; A_{o}u\mspace{14mu}{Cavity}\mspace{14mu}{Mass}\mspace{14mu}{Balance}}$where p=pressure in the small chamber and p_(a)=pressure in the largerchamber.

Combining the above two equations and introducing sound speed results in

${\frac{\mathbb{d}^{2}P}{\mathbb{d}t^{2}} + {\frac{a^{2}A_{o}}{L_{o}V}P}} = {{0\mspace{14mu}{where}\mspace{14mu} P} = {p - p_{a}}}$

A=sound speed

A₀=orifice area

L₀=orificelength

V=cavity volume

Therefore, P can support harmonic oscillations ω:

$\omega = \sqrt{\frac{a^{2}A_{o}}{L_{o}V}}$In an exemplary sensing device 10 with the following dimensions:

$\begin{matrix}{{d = {1 \times 10^{- 4}\mspace{45mu}{Cavity}\mspace{14mu}{deapth}}};} & {{h = {5 \times 10^{- 3}}}\;} & {{{Cavity}\mspace{14mu}{height}};} \\{{w = {1 \times 10^{- 3}\mspace{45mu}{Cavity}\mspace{14mu}{width}}};} & \; & \; \\{{d_{o} = {1 \times 10^{- 4}\mspace{34mu}{Orifice}\mspace{14mu}{deapth}}};} & {{h_{o} = {0.5 \times 10^{- 3}}}\;} & {{{Orifice}\mspace{14mu}{height}};} \\{{L_{o} = {0.5 \times 10^{- 3}\mspace{14mu}{Orifice}\mspace{14mu}{length}}};} & {{a = {500\frac{m}{s}}};} & {\rho = {10\frac{m}{s}}}\end{matrix}$the time for a cavity resonance is

$\omega = {\frac{1}{\tau} = {\sqrt{5} \times 10^{5}}}$

Another characteristic of an orifice in unsteady flow is the responsetime for the orifice to adjust to a step input of flow. If the orificeis represented by a fluid circuit, the basic dynamic equation is:

${{\frac{\rho\; L_{o}}{A_{o}}\frac{\mathbb{d}Q}{\mathbb{d}t}} + {\frac{\rho\; f\; L_{o}\overset{\_}{u}}{2A_{o}h_{o}}Q}} = {0 = {{I\frac{\mathbb{d}Q}{\mathbb{d}t}} + {R\; Q}}}$where Q = Volume  flow  rate; f = friction  factorthe solution for which is the following:

$\frac{Q}{Q_{i}} = {{{\mathbb{e}}^{- \frac{t}{\tau}}\mspace{14mu}{where}\mspace{14mu}\tau} = {{\frac{I}{R}\mspace{14mu}{and}\mspace{14mu} Q_{i}} = {{Input}\mspace{14mu}{Flow}}}}$Thus, using an exemplary average velocity of 10 m/s, friction factor of0.05, and the geometrical numerical values listed previously, we obtainthe value of τ as

$\tau = {{\frac{I}{R} \approx \frac{10^{5}}{2 \times 10^{7}}} = {5 \times 10^{- 3}s}}$So, for an exemplary engine rotating at 1800 rpm, the time for one cycleis 0.0333 seconds and the response time of the orifice is similar to thetime for the combustion of gases in the cylinder.

To predict the deflection of the beam 14 and moments and stresses in thesensing device 10, the following equations on strength of materials maybe used:

y = deflection$\frac{\mathbb{d}y}{\mathbb{d}x} = {\theta\mspace{11mu}{slope}}$${\frac{\mathbb{d}}{\mathbb{d}x}\left( {E\; I\frac{\mathbb{d}y}{\mathbb{d}x}} \right)} = {M\mspace{14mu}{Moment}}$${\frac{\mathbb{d}}{\mathbb{d}x}\left( {E\; I\frac{\mathbb{d}^{2}y}{\mathbb{d}x^{2}}} \right)} = {V\mspace{14mu}{Shear}}$${\frac{\mathbb{d}^{2}}{\mathbb{d}x^{2}}\left( {E\; I\frac{\mathbb{d}^{2}y}{\mathbb{d}x^{2}}} \right)} = {p\mspace{14mu}{Loading}}$For increased flexibility in the design of a pressure sensor, it isuseful to consider materials that are capable of having variableelasticity (E) and moment of inertia (I). When the equations above onstrength of materials are reformulated as coupled second order equationsfor ease of solution and application of boundary conditions, the resultsare as follows:

${\frac{\mathbb{d}}{\mathbb{d}x}\left( {E\; I\frac{\mathbb{d}y}{\mathbb{d}x}} \right)} = M$$\frac{\mathbb{d}^{2}M}{\mathbb{d}x^{2}} = p$

The solution embodied in the above equation may be implemented in acomputer code including tridiagonal matrix method, which is marched toconvergence by a time marching technique. FIGS. 2A-5C show the solutionof these equations for four different beam configurations: 1) a beamwith uniform cross-section and both ends with zero slope, 2) a beam withuniform cross-section and one end free, 3) a beam with decreasingcross-section and one end free, and 4) a beam with increasingcross-section and one end free. The cross-sections are varied by afactor of two in the beam height, and this causes the moment of inertia(I) to vary by a factor of eight. All scales in the figures arerelative, and it can be readily seen that the decreasing beam has themaximum deflection while the beam with both slopes being zero has theminimum deflection. The maximum moment is the same for all the freebeams, while the beam with zero slopes has a moment that is one half ofthe free end beams.

To determine and/or control the performance of an internal combustionengine, correlations between cylinder pressure variation and fuel burnedare developed. These correlations may be useful for determining sensordeflection and eventually the actual pressures in the engine cylinder.FIG. 6 shows typical results of pressure and fuel consumption for aninternal combustion engine. FIG. 7 shows the curve fit for pressure thatreproduce the engine data, and FIG. 8 shows the fuel consumption rate.The pressure curve can be distributed over an arbitrary angle about topdead center, and it has a variable maximum pressure. The curve fit maybe accomplished with the use of splines.

After the modeling is complete, the mask type is determined. Fordesigning the sensing device 10, the modeling data may be used to fixkey geometric features of the mask set design. Also, micro-machiningdesign rules as appropriate to MEMS and SNF are accounted for. Ahigh-definition print may be used as the masking technique.Alternatively, a chrome mask of the same type used in CMOS fabricationmay be used. The exact masking technique to be used may be chosen basedon resolution requirements, among other factors. For example, if featuresizes of less than 50 μm are required on the device, then the chromemask may be preferable.

The beam 14 may become fouled with use, adversely affecting the accuracyof the measurement of deflections and pressure. FIG. 9 illustrates apotential problem with fouling. As the gasket is clamped down, the metalpart of the gasket will flow into any orifice to ensure that thecylinder is correctly sealed. Thus, flow occurs into the cavity 12,changing the overall geometry of the sensing device 10.

To reduce this problem, the thickness of the beam 14 may beetched/machined down in the early stages of fabrication as shown in FIG.9. However, the effectiveness of this etch/machining depends on thelength to thickness ratio of the beam 14. The size of the gas flowpassage 16 between the beam 14 and the sidewall of the sensing device 10greatly affects the sensitivity of the sensing device 10. Making the gasflow passage 16 too small will result in quick fouling, while making ittoo large will reduce sensitivity of measurement. For a more accurateresult, an array of devices may be placed next to one another so thatthe readings from the devices may be extrapolated to obtain a correctreading.

The basic structure of the sensing device 10 may be split into two maincategories: fixed-free beam and fixed-fixed beam. Table 1 shows the beamdepths for each structural category of beams.

TABLE 1 Device beam depths (all 100 μm thick) Fixed-Free Fixed-FixedDevice (Cantilever) (Cantilever) Thinnest Section 400 μm 100 μmIntermediate Sections 600 μm 200 μm 900 μm 500 μm Thickest Section 2100μm  1000 μm 

The fixed-free device may be a simple cantilever whose depth is alteredfour times. FIG. 10 shows a structure where the beam 14 is a cantilever14 a. In normal operation, the beam 14 would move in the y-direction.The thickness of the finished device, measured in the z-direction intothe page, is 100 μm. As the depth of the cantilever 14 a (measured inthe y-direction) is 400 μm in this embodiment, the cantilever aspectratio is 4:1. If this is on the limit of allowable stress, that meansthe silicon at the base of the beam 14 will be at its UTS during normalengine operation.

To optimize the signal and hence device performance, the piezo-resistivesensing elements 22 that are used to monitor the strain are placed nearthe outer fiber of the beam 14. Optimization of the system is achievedby placing four piezo-resistive sensing elements 22 on the cantilever 14a (beam 14). With two sensing elements 22 being on the upper cantileversurface and two sensing elements 22 on the bottom (“upper” and “bottom”referring to FIG. 10 and FIG. 11), each pair will be experiencing eithercompression or tension according to which direction the beam 14 is bent.Having four piezo-resistive sensing elements 22 facilitates measurementas a bridge may be set up with each pair of sensing elements 22 placedopposite one another. The pairs may be wired up directly on the mask;however, this may not be done for the first run because it is desirableto obtain an initial measurement of each resistor value independentlyduring firing, due to the thermal gradient that is expected to formacross the beam.

FIG. 11 depicts a cantilever 14 a that is at the other extreme to thecantilever on the UTS limit that is shown in FIG. 10. The cantilever 14a of FIG. 11 is, simplistically put, a beam that is “certain” not tofail. The cantilever 14 a in FIG. 11 is 2100 μm deep (in they-direction) and has an aspect ratio of 21:1. Thermal errors would occurwhen firing with this device, as the temperature could be differentbetween the two sets of piezo-resistive sensing elements 22 (compressionand tension). The measures described above for temperature compensationmay be adopted to minimize thermal error.

The beam 14 may be a fixed-fixed beam structure instead of a fixed-freestructure like a cantilever. The fixed-fixed beam structure may alsohave four different beam thicknesses, as shown in Table 1. The valuesare, however, different as the strength of these designs is four timesthat of the fixed-free designs. FIG. 12 depicts a fixed-fixed beam 14 bhaving a depth of 100 μm, and FIG. 13 depicts a fixed-fixed beam 14 bhaving a depth of 1000 μm.

One of the difference between fixed-free and fixed-fixed beams is theposition of the piezo-resistive sensing elements 22. In both cases, thepizeo-resistive sensing elements 22 are placed as close to thehigh-stress areas of the beam 14 as processing will allow. In the caseof the fixed-fixed beams, this configuration is easy to make asconnection tracks can be run down along both sides of the die. It ispreferable not to place the piezo-resistive sensing elements 22 on theouter fiber of the device, as the sensing elements 22 are photosensitiveand doing so could give inaccurate reading during firing.

Device Fabrication

The sensing device 10 may be fabricated by using a process that is avariation on the well-known process for producing ultra-thin straingauges. Like the engine pressure sensor, these strain gauges are madefrom a semiconductor material, preferably a single-crystal silicon, thatis doped in strained regions to produce piezo resistors. The fabricationprocess may use six masks, although this is not a limitation of theinvention. Preferably, the sensing device 10 is etched out of a puresilicon wafer.

The sensing device 10 may be fabricated using either SOI or plain (B′)wafers. SOI wafers have a buried layer of silicon dioxide. The SOIwafers have been fabricated with device layer thickness: some having adevice layer of approximately 100 μm and others having a device layer ofapproximately 20 μm. “Device layer thickness” refers to the amount ofsilicon on the buried oxide. The plain wafers are single crystal siliconwith no oxide layer. The purpose of the oxide is to act as an etch stopfor the bulk micromachining. As such, the plain wafers have to have atimed etch to define the mechanical structures. This in itself is nottechnically difficult but the oxide also acts as an etch stop when thehandle silicon is removed. A combination of wafer grinding, polishing,and back DRIE etching give a high degree of control on the process thatallows the device to be released from the plain wafer.

FIG. 14 shows a first mask 50 of the set of masks used to fabricate thesensing device 10. The first mask 50 depicted in FIG. 14 is a shallowKOH mask. The KOH mask is useful for producing a relatively shallow etchon the surface of a device. The shallow etch produces small pits for theflex interface to fit into, ensuring alignment and decreasing thepackage thickness. The shallow etch also removes material from thethickness of the beam 14. Using the first mask 50, the etch time isabout five minutes at about 80° C. As KOH attacks the photoresist, asilicon dioxide layer of about 500 nm in thickness is grown on the waferbefore patterning. The first mask 50 is then put down and the wafer isetched in HF to remove the oxide. During the KOH etch, it is theremaining oxide that actually masks the wafer. After the KOH etch, theremaining oxide is removed and the wafer is decontaminated frompotassium prior to the CMOS processing.

FIG. 15 shows a second mask 52 of the set of masks used to fabricate thesensing device 10. The second mask 52 defines the piezo-resistivesensing elements 22. As With the KOH mask, an oxide is initiallydeposited and etched using HF. The oxide masks the device duringimplantation. Unlike the KOH mask, this oxide is not removed after theimplantation. During patterning, the oxide is removed from those partsof the mask that will be bulk micromachined in the final etching stage(FIG. 19) using DRIE. Hence, the oxide layer is etched from the areaaround the beam 14.

FIG. 16 shows a third mask 54 for the contact etching. This maskrepresents the highest resolution required in the device processing.Contact etching and contacting to the implants is inherently a complexprocedure. Thus, if the high resolution is about 2 μm, the window may beopened up to a 10-μm range. In addition to giving a much largercontacting area, the mask allows for additional doping of the contactpoint of the resistor if it is deemed necessary. The third mask 54 isexposed after the wafers have been deposited with 2-μm oxide. The oxideis then etched using HF.

The bottom left corner of FIG. 16 shows that two windows 55 a, 55 b areopen to the silicon. In FIG. 15, only the smaller of the two windows(shown as window 53) is doped. The windows give two terminals in thefinished device: one enabling direct contact to the silicon (e.g., thelarger pad) and the second one offering an effective p-n diode. Theseterminals can be used to check the electrical properties of eachindividual device. The direct silicon contact can also be used to biasthe base silicon in relation to the piezo resistor implants, providingan effective p-n junction between resistors. Such a system is lesslikely to leak current and experience cross talk during measurements.

FIG. 17 shows an aluminum mask 56 for the 400 μm fixed-free sensingdevice (see FIG. 10). There may be a “border,” i.e. the aluminum layer,around the sensing device 10 in all masks (FIGS. 14-20). The border iseventually removed of material so that the device can be “punched out”of the wafer and released. Instead of using the border, the device maybe released from the wafer by sawing with a diamond blade. However, dueto the thinness and fragility of the MEMS devices, sawing may not alwaysproduce the desired result.

After the piezo-resistive sensing elements 22 are implanted into thesilicon and the contacts are opened through the protective silicondioxide layer, metal tracks are run to the resistors. The metal tracksusually contain aluminum, although other metals may be used e ifdesired. It may be desirable to make aluminum tracks thicker than theywould expected to be for devices of similar scale and geometry becausethe sensing device 10 will be operating in a harsh enviromnent insidethe cylinder. In all other aspects of the design, the aluminum mask is astandard.

FIG. 18 shows a fourth mask 58 for forming the Low TemperautareOxidation (LTO) contact pads. After aluminum patterning, the devices aredeposited with a protective LTO layer to protect the aluminum from theharsh environment. The LTO is patterned to open up the contact window tothe aluminum. In the exemplary embodiment, the fourth mask also definesthe cantilever structure as the oxide is removed prior to the DRIE etch.

FIG. 19 shows a fifth mask 60 for bulk removal of silicon to define thephysical structure of the sensors. The DRIE is the finalphotolithography stage. Non-standard 10-μm thick photoresist is used.Although the 10-μm photoresist does not retain the same definitionproperties of thinner resists, it is likely to be able to withstand theharsh plasma used during DRIE.

FIG. 20 shows the layout of a sixth mask 62. As shown, the layout ismodular and allows a “common” interface tape method to be used to jointhe engine sensor to its packaging. Hence, all devices on this mask setcan fit onto standard 3M-microflex tapes. In addition, the mask set mayinclude numerous devices. Thus, paddles, strain gauges, and pressuresensors may be fabricated all on the same mask.

FIG. 21 shows an optional test cell 70 that may be fabricated with thesensing device on the same mask set. Working clockwise from the topright in FIG. 21, there are:

-   -   a four-point probe tester 72 used to gauge the implant/piezo        resistor and its conductivity;    -   a plain resistor 74 with much larger geometry than that of the        engine sensors' resistors;    -   a set of intertwined resistors 76 to check for leakage    -   a set of 20 resistors 78 wired up in series, used to examine        contact resistance;    -   a pair of cantilevers 80 that can be bent in the plane normal to        the mask, used to measure the gauge factor of the piezo        resistors;    -   a direct and diode connection 82 to the device layer silicon,        exactly as used in the actual device; and    -   a single cantilever 84 set tangentially, used to check that the        orientation on the crystal is correct (e.g., by comparing        results with the gauge factor measured above)

The devices thus described are preferably processed from Silicon OnInsulator (SOI) wafers. Properties of SOI wafers are summarized in Table2. The devices operate through the bending of the beam 14 that has to beshallow-etched so that it does not foul on the gasket or cylinder head.during bending. The beam is defined through Deep Reactive Ion Etching(DRIE). The fabrication involves doping the piezo resistors into thesilicon, patterning out the aluminum contacts and dry etching out thedevice silicon to the oxide buffer layer. The handle can then be removedby a combination of grinding, polishing, and DRIE.

TABLE 2 Properties of wafers Number and type of Wafer Device layer OxideHandle wafers diameter thickness thickness thickness 12 SOI 100 mm 20 μm2 μm 350 μm 12 Standard (b′prime) 100 mm N/A N/A 550 μm

The sensor may be fabricated using the process explained in U.S. patentapplication Ser. No. 11/048,462 filed on Jan. 31, 2005, which is herebyincorporated by reference in its entirety for all purposes. The processdescribed in the pending application uses SOI wafers to produce finishedthree-dimensional structures (MEMS devices) that are 100 μm thick.

FIG. 22 depicts the process schematically, as process 100. There arenumerous steps to the process and as can be seen in the right-mostcolumns of FIG. 22, the process is adaptable. This allows processvariation at set points in the design of a device. The process startsinvolves:

-   -   providing a semiconductor substrate and forming a cavity (step        102).    -   a shallow KOH etch for interface (step 104). A surface micro        machining wet etch that forms shallow pits into which the device        interface can be aligned.    -   piezo resistor implant that forms the piezo-resistive sensing        elements 22 (step 106). Generally, this operation is performed        off-site.    -   contact oxide-etch. After implant, an oxide layer is deposited        to insulate the piezo-resistors. This layer has to be dry-etched        in plasma so that electrical contact can be made to the        resistors.    -   aluminum (sputter, mask & pattern) (step 108). This aluminum        makes contact to the resistors through the contact windows and        is patterned to form tracks to contact pads. In most processes,        these pads are for wire bonding. However, in the case of        in-cylinder sensor, this device will interface directly with        flex-type tape.    -   DRIE (step 110) defines the actual mechanical structure. It uses        a technique known as the “Bosch process,” which allows silicon        to be etched with high aspect ratios (up to 100:1). The process        being anisotropic, it allows the structure to be etched normal        to the wafer surface.    -   Back Grind and DRIE (step 112). Once the structure has been        defined, it requires releasing from the background layer of        silicon (known as the handle). This is achieved by back-grinding        the wafer. Essentially identical to the macro grinding process,        it allows for bulk removal of silicon. The back-grind falls        short of the oxide interface with the device and is completed        with a much slower but more controlled plasma etch.    -   the devices are released from the handle and attached to        electrical interface for the flex material (step 114). This        release process involves dropping the wafer into acetone and        sieving the devices out.    -   connecting to the flex tape (step 116). Electrical connections        are made to the obtained flex tape.

Device Packaging and Mounting

FIG. 23 shows a flex tape used to interface the sensing device 10. Theflex tape allows the sensing device 10 to communicate with outsidedevices. Typically, cylinder head gaskets are between 1.0 mm and 1.2 mm.Therefore, it is desirable for a MEMS device that will be placed in thecylinder to have a thickness of less than 50 μm to avoid adverse effectson the gasket performance. A 25-μm thick Kaptong® tape that is patternedwith 15 μm of copper (and maybe 2 μm of gold capping) interconnects.

The pressure/temperature sensor can be designed to mate to the flex thatis already in use, to form a flexi-mounted device. The flexi-mounteddevice will then be mounted into the cylinder head of a small glow plugengine (of the sorts used for model airplanes).

Interface between the sensing device 10 and other devices (not shown)utilize some electronics to give a voltage output that is proportionalto beam deflection. This voltage output is not exactly the same as adirect pressure output. To obtain these outputs, an integrator anddifferentiator may be incorporated, as these values are a function oftime. FIG. 24 shows a bridge circuit that may be used to measure beamdeflection. More specifically, the bridge circuit is useful foramplifying the signal from the bridged piezo resistors. In addition tothe base measurement, torsion strains put upon the beam during firingmay be examined. The examination may include examining the ratio of thetotal of the resistance fluctuation from strain.

The flex material or the flex tape on which the sensing device 10 ismounted is flexible and shows excellent heat tolerance. The flexmaterial may be custom-ordered from companies such as 3M, or implementedwith a commercially available part. Table 3 shows material properties offlex components, which are commercially available. An external devicecommunicates with the sensing device 10 while the sensing device 10 isin the cylinder head gasket. 3M has been contracted to provide microflextape for this application. Ordinarily, in a high-temperatureapplication, this type of tape (made from Kapton) would not be suitable.The flex pattern is similar to that shown in FIG. 25. In the exemplaryflex pattern shown in FIG. 27, two planar strain gauges and a LinearPolarization Resistance (LPR) corrosion sensor are fitted to the flex. A40-pin Zero Insertion Force (ZIF) connector is used to interface theflex to a PCB. The overall length of the flex is 17 cm (4.28 inches).

TABLE 3 Flex component material properties Material Properties Polyimidesubstrate (DuPont Mechanical properties: Kapton Type E) Modulus 800 KPSICTE = 17 ppm/° C. CHE = 9 ppm/% RH Shrinkage = 0.03% (200° C.) Coppercircuitry Metallurgical properties: Electrodeposited (high ductility)99.9% purity IPC-TM-659 method 2.3.15 Tensile Strength 40-60 Kpsi ASTME-345-86 Elongation 5% min. ASTM E-345-86 Coefficient of Expansion 1.7 ×10⁻⁵ 20° C. < T < 100° Hardness 120 HK +/−20 ASTM-578-87 Gold & Nickelplating on the Typical Nickel thickness 0.4-1.25 um as tested by x-Raycopper circuitry Fluorescence Typical Gold thickness 0.2-3.2 um 99.9%pure gold Epoxy protective layer (Asahi Note: There are Polymideadhesiveless based protective layers other Chemical Research Laboratorythan Epoxy on the market. These materials handle higher than epoxy SuperResist CCR-232 GF No. 6) operating temperatures. These “covercoats” arenot in product currently at 3M but could be applied to sensor in labsetting for evaluation. (See vendors listed below)The electrical interface of the microflex uses an adhesive loaded withnickel particles. The adhesive comes in tape form and is epoxy-based.The electrical interface is thus not suitable for applications above120° C. However, as the sensing device 10 is to fit in the head gasketand will be under pressure during elevated temperature (above 120° C.),any plastic flow of the adhesive will lead to improvement of theelectrical interface.

Device Operation

Table 4 lists some operational criteria for a cylinder head sensor.

TABLE 4 Operating Criteria for an in-cylinder pressure sensor Continuoussensor Pressure Over temperature Frequency Temperature range pressurerange Response Sensitivity SNR Lifetime 0 to 30 kpsi ×2 of −40° C. to350° C. 0.01 Hz to ±0.03% 2000:1 0.5 × 10⁹ pressure 30 kHz @ 15 kHzcycles rangeConsidering a MEMS sensor fabricated from a single-crystal silicon, thesensing device 10 is able to meet most, if not all, of the criterialisted in Table 4. Referring to Table 4, the pressure range values arerelevant to a cavity flow device where reed ductility determines themeasurable pressure range and sensitivity of measurement. Generally,greater the measurable pressure range, the less sensitive themeasurements will be.

The limiting operational temperature range of the sensing device 10 isthat at which the thin film aluminum that forms the interconnects fromthe piezo resistor sensing elements 22 to the bond pads 18 begins todegrade. This degradation typically happens in the 600° C. range andhence the device will be more than capable of handling the temperaturesfounding a typical Otto cycle engine. A flex for connecting the deviceto the electronics that can withstand the hottest temperature inside acylinder is manufactured by 3M. This flex material may contain, forexample, DuPont's E-Film that has a long-term stability of 300° C. and ashort-term stability of about 400° C.

Frequency response can be easily adapted to suit the 30 kHz criteria.Presently, MEMS cantilever devices with resonance frequencies in the MHzrange are available and such construction poses no technical challenges.

Temperature sensitivity in the 10⁻³ degree range requires theincorporation of a p-n diode at the combustion face.

The Signal-to-Noise Ratio (SNR) may be challenging for petrol enginesdue to the RF noise produced by the ignition system. Other than thisexternal noise source, the system electronics may be designed such thatwhite noise from the piezo resistors is filtered out.

The beam 14 is virtually solid state. As such, the lifetime reliabilityof the system is well in excess of that of the gasket and even theengine.

Using the sensing device 10 of the invention, failure in valves, pistonrings, or the cylinder head gasket can be identified. The sensing device10 fits in the cylinder head adjacent to the gasket with the sensingelements 22 exposed to the combustion process in the cylinder. Thesensing device 10 may be incorporated into existing engines or builtinto new engines. By using MEMS technology for device fabrication, onecan keep the unit cost of each sensing device 10 (and thus the unit costof the resulting sensor) low.

Although the invention has been described with reference to the aboveexample, it will be understood that modifications and variations areencompassed within the spirit and scope of the invention. Accordingly,the invention is limited only by the following claims.

1. A sensing device comprising: a planar substrate characterized by aplane; a beam formed on the substrate, the beam being capable ofdeflecting in the plane of the substrate according to differentpressures applied to the beam by a flow of gas; and piezo-resistivesensing elements coupled to the beam, the piezo-resistive sensingelements detecting beam deflection and generating an electrical signalcorresponding to the beam deflection.
 2. The sensing device of claim 1further comprising a cavity formed in the substrate, wherein the beamdeflects according to difference between pressure inside the cavity andpressure outside the cavity.
 3. The device of claim 1, wherein thesubstrate is a silicon substrate and the piezo-resistive sensingelements are doped into the silicon substrate.
 4. The device of claim 1,wherein the piezo-resistive sensing elements form a first pair ofpiezo-electric sensing elements, further comprising a second pair ofpiezo-resistive sensing elements for detecting temperature.
 5. Thedevice of claim 4, wherein the two pairs of piezo-resistive sensingelements are formed on the same surface of the substrate.
 6. The deviceof claim 4, wherein the two pairs of piezo-resistive sensing elementshave the same crystal orientation.
 7. The device of claim 1 furthercomprising: bond pads on the substrate; and aluminum connections betweenthe piezo-resistive sensing elements and the bond pads.
 8. The device ofclaim 1 further comprising a piezo resistor formed on the substrate fortemperature measurement.
 9. The device of claim 1 further comprising ap-n diode formed on the substrate for temperature measurement.
 10. Thedevice of claim 1, wherein the beam has a fixed end and a free end, thefixed end being coupled to the piezo-resistive sensing elements.
 11. Thedevice of claim 1, wherein the beam is a cantilever.
 12. The device ofclaim 1, wherein the beam has two fixed ends.
 13. The device of claim 1,wherein the beam has a uniform cross-section through its length.
 14. Thesensing device of claim 1, wherein the device is no larger than about 10cm×about 10 cm×about 100 μm.
 15. A sensor comprising: a flexiblematerial having an electrical interface; a sensing device mounted on theflexible material, the sensing device including: a planar substratecharacterized by a plane; a cavity formed in the substrate; a beamformed on the substrate, wherein a gas flow passage is formed betweenthe beam and the cavity, the beam being capable of deflecting in theplane of the substrate according to a flow of gas in the gas flowpassage proportional to a difference between pressure inside the cavityand pressure outside the cavity; piezo-resistive sensing elementscoupled to the beam, the piezo-resistive sensing elements detecting beamdeflection and generating an electrical signal corresponding to the beamdeflection; and bond pads electrically coupled to the pair ofpiezo-resistive sensing elements; wherein the bond pads are electricallycoupled to the electrical interface on the flexible material.
 16. Amethod of fabricating a sensing device, the method comprising: providinga planar substrate characterized by a plane; forming a beam on thesubstrate that is capable of deflecting in the plane of the substrateaccording to pressure applied to the beam; and doping a portion of thesubstrate to form piezo-resistive sensing elements coupled to the beam,the piezo-resistive sensing elements detecting beam deflection andgenerating an electrical signal corresponding to the beam deflection.17. The method of claim 16 further comprising etching the substrate toform a cavity so that the piezo-resistive sensing elements detectpressure difference between inside the cavity and outside the cavity.18. The method of claim 16 further comprising doping a different portionof the substrate to form a temperature-sensing piezo-resistor.
 19. Themethod of claim 16, wherein the piezo-resistive sensing elements are afirst pair of piezo-resistive sensing elements, further comprisingdoping another portion of the substrate to form a second pair ofpiezo-resistive sensing elements for temperature measurement.
 20. Themethod of claim 19 further comprising forming the two pairs ofpiezo-resistive sensing elements on the same surface of the substrate.21. The method of claim 19 further comprising: forming a bond pad on thesubstrate; and electrically coupling the bond pad to the pair ofpiezo-resistive sensing elements.